Narrow Antenna Beam SAR
According to the SAR principle, numerous radar pulses are transmitted along a flight path. The responses from the ground are recorded and stored along the flight path. Thereby, the SAR system can be compared to a stationary array antenna consisting of as many array elements as the number of pulses emitted along the path. The flight path builds a “synthetic aperture”.
In the early days of synthetic aperture radar, the antenna had a relatively restricted beam width and was operating at a restricted signal bandwidth, for instance in the microwave range. For stripmap SAR, the ground surface could be depicted using Fast Fourier Transform (FFT) algorithms for the image formation. This method requires only a limited computing processing power.
OMNI Antenna Beam SAR
More recently, wide bandwidth omni-directional antenna SAR systems have been introduced. Airborne Low Frequency Synthetic Aperture Radar (LF-SAR) operating in the VHF and UHF bands is becoming an important emerging technology for wide area surveillance and target detection in foliage. A VHF synthetic aperture radar system denoted CARABAS (Coherent All Radio Band Sensing) SAR system has been described in U.S. Pat. No. 4,965,582 and U.S. Pat. No. 4,866,446. An ultra wide band coherent radar system has been disclosed U.S. Pat. No. 6,072,420.
The function of the CARABAS system shall briefly be described in the following.
The antenna diagram of the two boom CARABAS system, comprising a right antenna AR and a left antenna AL has been illustrated in FIG. 1, whereby the respective diagrams for left, 11, and right, 12, have been indicated. It is noted, that the each respective diagram is asymmetric because of the shading property of the adjacent antenna. A so-called main lobe ML and a backlobe BL appears for each antenna.
In the following, an account of the circuitry and the signal processing properties of the known CARABAS system shall be given with reference to FIGS. 2, 3 and 4.
The known CARABAS system, as shown in FIG. 2, comprises an arbitrary signal generator AWG adapted to operate over a wide frequency range such as in the interval of 20 MHz-100 MHz, for instance emitting radar pulses using pulse compression techniques, such as generating a signal pulse with linearly increasing frequency, a so-called chirp. The system also comprises a transmit power amplifier TX, a switch SB, for feeding the generated pulse into a conventional additive beam forming transmit unit, ABF_TX.
By operating the switch, the antenna pair is excited to transmit a beam tilting more to one side of the aircraft than to the other. When the switch is flipped, the beam is reversed and tilts in the opposite direction with respect to left/right symmetry of the aircraft. For one position of the switch SB, the additive beam-forming transmit unit will produce a delayed signal at no power loss to a left antenna, AL, together with a non-delayed signal to a right antenna, AR, for transmitting the radar pulse to the left side of the platform.
The two antennas AR and AL are moreover connected to transmit/receive switches SR and SL. The switches are periodically steered in synchronization such as to either pass the signal being fed by arbitrary waveform generator AWG and a power amplifier TX or to receive the corresponding radar response.
In the receive path, an additive beam-forming receive unit, ABF_RX, is provided for amplifying signals coming from a desired direction and subduing signals coming from the opposite direction via the two antennas. This unit may be based on delay circuitry as is the case for ABF_TX and provides simultaneously a separate “resolved” left-hand signal and a separate “resolved” right-hand signal.
By issuing a pulse while directing the antenna system by means of ABF_TX to the right and subsequently receiving a pulse directing the antenna system to the right by means of ABF_RX, a gain in sensitivity separation is accomplished, the latter being indicated by the enhanced antenna diagram 13 in FIG. 1. As will be understood from the antenna diagram of FIG. 1, the directional separation is not perfect, why both left and right hand signals are “polluted” with signals stemming from the undesirable, i.e. back-lobe side, of the antenna system. It is seen that the gain in separation is highest for directions orthogonal to the antenna, while the separation is less effective for directions at slant angles with respect to the antennas.
The additive beam forming receive unit, ABF_RX, provides the respective left-hand side detected signals and the respective right-hand detected signals to respective memory bank, MEM.
An operation cycle T1-T4 of the above circuit shall now be explained with reference to FIG. 3, whereby the flying platform is moving over a featureless landscape except for two identical objects that are arranged symmetrically to a linear flight path. The signal magnitudes at N1 of FIG. 2 have been indicated in FIG. 3.
At time T1, a radar pulse is emitted to the left. At time T2, the antenna system is pointed to the left, whereupon a signal echo is received originating from both objects. The resulting signal is imagined as consisting of the following power contributions. P_IN refers to the signal contribution, which is caused by internal noise in the receiver section, P_EN refers to the external noise, P_L refers to the signal contribution originating from the left object and P_R refers to the signal contribution originating from the right object (the patterns (hatched) of FIG. 3 are used consistently throughout the application so as to identify these signals). The index LTX, LRX refers to a pulse being issued to the left and received as the antenna system is pointing left.
At time T3, a pulse is transmitted to the right and at time T4, the antenna system is receiving the responses while the antenna system is pointing right.
At the first measurement sequence T1 and T2, the resulting signal which is used for the imaging process is P(2)=P_IN (2)+P_EN(2)+P_L(2)+P_R(2). Since P_L(2)>P_R(2) and the sum of the external noise and the internal noise, P_IN(2)+P_EN(2), are substantially constant over time, the resulting signal P(2) will be indicative of the properties of the left object although the “ghost” response from the right object will also appear due to the poor separation.
At the second measurement sequence T3 and T4, the properties are opposite and mainly the right object will contribute to the total signal. P_R(4)>P_L(4) so that the signal response from the right side object will contribute more to the subsequent image generation. Hence, the resulting image will not be free from the noise stemming from objects on the back-lobe side.
It should be understood that the emitted beams to each respective side need not be mirror copies of each other but must—more importantly—be known to some degree of accuracy.
Synthetic Beamforming
For an omni-directional antenna, a singular emitted radar pulse will produce a spherical shell shaped wave, propagating at the speed of light from the aperture position. The corresponding pulse, when echoed, will undergo a change in Doppler shift depending on the angle to the direction of movement, which in this case is parallel to the antenna. At an angle perpendicular to the antenna the Doppler shift is zero.
The net signals—returning now to FIG. 2, are fed into respective memory banks MEM for the right side and the left side paths—are signal processed by well-known methods of coherent radar (e.g. Doppler beam sharpening), in synthetic beam forming units, SBF to obtain synthetic beams with respect to an assumed stationary ground. For an emitted given pulse or chirp, responses are associated and stored in both range bins, i.e. a resolved range interval, and resolved Doppler bins, i.e. directional bins albeit with a right left ambiguity.
In FIG. 4, the Doppler bins are illustrated as a number of leafs in the antenna diagram, whereby responses for a given direction is ambiguous with regard to left and right, e.g. leaf 21 and leaf 22. Using the left/right beam-forming directivity enhancement produced by ABF_TX and ABF_RX, the responses from the left, respectively, the right, is resolved/stored in synthetic beam forming unit SBF. The “right side” resolved synthetic beams have been shown under reference numeral 23.
The data of the respective synthetic beam forming units SBF are provided to an image generation unit, IMAG_GEN, which produce the resolved SAR picture.
SAR Algorithm
As shown in FIG. 4, during flying over the synthetic aperture length, L, a plurality of pulses, P, is emitted. During the same aperture length, a plurality of signals are received and arranged in range bins and Doppler bins.
Hence, the operation cycle T1-T4 is done starting from time t1, in FIG. 4. At time t2 the cycle is repeated.
As mentioned above, a singular emitted radar pulse will produce a spherical shell shaped wave, propagating at the speed of light from the aperture position. The corresponding responses to the radar pulse that are sampled and ordered in range bins will represent responses of arcs intersecting with ground. Hence, the given response from a single pulse for a given range bin will depend on the sum of responses along the arc on the ground, which again will depend on the ground reflectivity in all points along the arc. This has been illustrated in FIG. 4, where a pulse is emitted at t1 and whereby echoes from a certain distance corresponding to an arc, R1-R2, intersecting with ground will be produced. Please note that FIG. 4, and the other diagrams illustrating magnitudes, are only schematic.
Issuing multiple pulses along the aperture length, P/L, c.f. FIG. 4, and sampling the responses in range bins (and Doppler bins) provides a pattern with multiple arcs intersecting with one another (at directions for which the response can be resolved from the corresponding Doppler bin). From the accumulated ground reflectivity stemming from the intersecting arcs, (at the given directions also using the Doppler bins) the ground reflectivity in a given point on the ground can be resolved.
It is noted that the principle of SAR image generation works without making use of Doppler data. The amount of data processing, however, can be considerably reduced taking Doppler data in account.
Based on the stored data, the image generation unit, IMAG_GEN is performing the image generation on the respective right side, R, and left side, L, data.
Various methods may be used for the image generation process, for instance a method referred to as the “back projection method”, described in “Synthetic-Aperture Radar processing using Fast Factorised Back-Projection” L H. Ulander, H. Hellsten, G. Stenström, IEEE Transactions on Aerospace and Electronic Systems Vol. 39, no. 3, July 2003.
These methods shall not be explained further here, as they are well known in the art. The Image generation performed in image generation unit shall therefore not be dealt with further.
Omni directional antenna based SAR systems (backprojection SAR) have enhanced resolution considerably. But omni-directional antennas will only provide a clean, ghost free, high resolution SAR image when the input measurements or data is sufficiently separated as to left/right ambiguities.